Am I correct in assuming that, in normal operation, there is no venting going on, right?

Correct. The cap is simply a relief valve for safety purposes. I feel as if people are under the assumption that the cooling system operates at 2 bar. It's nominal operating pressure is likely much less. The system is always completely sealed unless it's exposed to atmospheric pressure either through the cap (which houses the relief valve) or through a leak somewhere in the system.